Fiberoptic telecommunications are continually subject to demand for increased bandwidth. One way that bandwidth expansion has been accomplished is through dense wavelength division multiplexing (DWDM) wherein multiple separate data streams exist concurrently in a single optical fiber, with modulation of each data stream occurring on a different channel. Each data stream is modulated onto the output beam of a corresponding semiconductor transmitter laser operating at a specific channel wavelength, and the modulated outputs from the semiconductor lasers are combined onto a single fiber for transmission in their respective channels. The International Telecommunications Union (ITU) presently requires channel separations of 50 GHz, or approximately 0.4 nanometers. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Improvements in fiber technology together with the ever-increasing demand for greater bandwidth will likely result in smaller channel separation in the future.
Transmitter lasers used in DWDM systems have typically been based on distributed feedback (DFB) lasers operating with a reference etalon associated in a feedback control loop, with the reference etalon defining the ITU wavelength grid. Statistical variation associated with the manufacture of individual DFB lasers results in a distribution of channel center wavelengths across the wavelength grid, and thus individual DFB transmitters are usable only for a single channel or a small number of adjacent channels. Continuously tunable external cavity lasers have been developed to overcome this problem.
The trend towards smaller channel separation and the advent of channel selectivity in transmitter lasers has given rise to a need for greater accuracy and control in the positioning of tunable elements associated with transmitter lasers. As tunable elements are configured for narrower channel separation, decreasing component tolerances and thermal fluctuation become increasingly important. Non-optimal positioning of tunable elements results in spatial losses and reduced transmitter output power.
An etalon is a widely used tunable element that acts to filter specific frequencies of light. An etalon is a type of interference filter in which the intensity of transmitted light is dependent on its wavelength. An etalon is generally comprised of two partially reflective parallel surfaces spaced a distance apart and separated by a material. The material may be glass plates or similar material having a reflective or partially reflective coating and the partially reflective surfaces are the interface of that material with the surrounding environment. When collimated light of a certain wavelength passes through the etalon some of the light is reflected between the surfaces. Constructive and/or destructive interference of light beams within the space between the surfaces occurs and attenuates unwanted wavelengths preventing them from passing through the etalon.